The present invention relates to a method of metallizing the surface of non-oxide ceramics. In particular, the present invention relates to a method of metallizing non-oxide ceramics at low cost, thereby enabling large-scale production of the same on a commercial basis.
The "high-melting point metal" method, a common technique for metallizing oxide ceramics such as alumina, has also been applied to non-oxide ceramics and metallized AlN substrates fabricated by such applied technique known as "tungsten metallizing method" have been developed and sold on the market.
A method is also known in which a Ag paste, Ag-Pd paste or Cu paste is printed or otherwise coated on a ceramic alumina substrate and its surface is metallized by firing either in an oxidizing atmosphere (in the case of Ag or Ag-Pd paste) or in a non-oxidizing atmosphere (in the case of Cu paste). As a modification of this process, pastes applicable to non-oxide AlN ceramics have been developed and gained increasing acceptance in the industry.
A more recently developed method for metallizing ceramics comprises roughening a surface of ceramics, activating the roughened surface with palladium, and depositing a metal plate by electroless plating. A method has also been developed in which an active metal is coated on the surface of a ceramic material and heated in an inert atmosphere so that the ceramic material reacts with the active metal to have its surface metallized.
In the case where the metallized product is to be used as a high-frequency circuit substrate, metallization is effected by vacuum-evaporating a metal on the surface of the ceramic material. This method has proved to be effective for both types of ceramics, oxides or non-oxides.
As it turned out, however, the prior art methods have suffered the following problems when they are applied to metallization of non-oxide ceramics. In such methods as the tungsten metallizing method which perform metallization using high-melting point metals, a paste of high-melting point metal is coated onto a green sheet, which then must be fired at a temperature as high as 1,200.degree.-1,800.degree. C. either in vacuum or in an inert atmosphere and this increases both the initial and running costs. Further, the firing of the green sheet is disadvantageous in that the dimensional precision of the metallized surface after firing deteriorates appreciably on account of the thermal shrinkage which accompanies the firing. In addition, tungsten and molybdenum commonly employed as high-melting point metals are expensive and their electric conductivity after firing is low compared to other metal elements.
The use of Ag and Ag-Pd pastes also involves problems; first, the strength of their adhesion to non-oxide ceramics is not as high as the value exhibited by conventional 96% class alumina substrates, and second, migration occurs. Further, the oxidizing atmosphere which is necessary for firing Ag and Ag-Pd pastes causes either oxidation of the non-oxide ceramic material per se or occasional failure to accomplish thorough firing on account of the creation of a reducing atmosphere in the paste-coated area during firing because the non-oxide ceramic material is burnt and decomposed during firing. As a further problem, the pastes have incorporated therein an oxide or other ceramics that are commonly referred to as "glass frit" in order to provide improved adhesion to the non-oxide ceramic material to be metallized and such "glass frit" prevents silver from exhibiting its inherent electric conductivity.
In contrast to Ag and Ag-Pd pastes, the Cu paste which is fired in a non-oxidizing atmosphere is free from the problems associated with the combustion and decomposition of the non-oxide ceramic material. However, the Cu paste also contains a glass frit and after firing, it displays an extremely low electric conductivity as compared to pure copper. Further, the strength of adhesion of the Cu paste is generally lower than that of Ag and Ag-Pd pastes.
The method of metallization that employs electroless plating comprises the steps of roughening the surface of a ceramic material, activating the surface with Pd and applying a plate by electroless plating. In this method, the adhesion of metal plate is weak since it is achieved only by the anchor effect provided in the roughening step. Further, the roughened surface must be activated with a Pd salt and the use of expensive Pd is costly and unsuitable for large-scale production. As another problem Pd which is left between the ceramic substrate and the metal plate must be removed before an electric circuit is formed by etching.
The "active metal method" has the disadvantage of heating the active metal until it melts so that it can react with the surface of the ceramic material. Active metals generally have high melting points and are active, so that they must be heated either in vacuum or in an inert atmosphere. This necessitates the use of large facilities and the operation must be done at high running cost. Further, the metallic surface provided by metallization with active metals are generally low in electric conductivity and wet poorly with solder, so the metallized surface must be later plated.
Attempts have been made to lower the melting points of active metals by alloying them with metals such as Ag, Cu and Ni but the use of active metals still presents the aforementioned problems associated with the heating atmosphere. If vacuum is to be used, it takes such a long time to evacuate the system that the operational efficiency is not so high as to warrant continuous production. Inert gases such as argon are too expensive to be discharged into air atmosphere.
Metallization of non-oxide ceramics by vacuum evaporation suffers the problem of limited applicability since the evaporated metal layer has low adhesion strength. This method is also unsuitable for large-scale production because of several reasons including the low production rate accompanying evacuation of the system and the loss of starting materials due to evaporation. In addition, prolonged evaporation is necessary to provide a reasonable thickness of metal layer and this further reduces the production rate of the overall operation.